[FLINK-39527][test] Migrate flink-end-to-end-tests to JUnit 5

[mateczagany]

What is the purpose of the change

Update flink-end-to-end-tests module to use only JUnit 5 instead of JUnit 4

Brief change log

• Migrated assertions to use AssertJ
• Limited visibility of classes/methods/fields where I could
• Upgraded Prometheus E2E test to use newer release, to make it work on ARM-based MacOS
• Replaced JUnit 4 runners with JUnit 5 extensions, such as FlinkResourceExtension and DownloadCacheExtension

Verifying this change

• The PR should pass all CI checks

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

[flinkbot]

CI report:

• 78e9cc5 Azure: SUCCESS

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